The invention relates to an improved drive arrangement for an electrically driven vehicle in which interference which is emitted by an electric motor via shafts for driving the motor vehicle is reduced.
Electric motors for driving a motor vehicle are known in the prior art, for example in the case of vehicles having a so-called hybrid drive, which has an internal combustion engine and an electric motor, and completely electrically driven vehicles, which are driven only by an electric motor. An electric motor typically comprises a stator with a plurality of stator windings. The rotor can have permanent magnets, electrically excited magnets or a cage.
The stator windings are usually driven by an inverter. The inverter does not generate an ideal signal profile but a signal profile having jumps and interference spikes. Such jumps and interference spikes cause so-called wave currents or wave voltages in the electric motor. These wave currents can be propagated through the gearbox to the drive shafts, from which interference fields are output. It goes without saying that such interference fields are undesired.
DE 11 2010 004 938 T5 discloses a retrofit kit for grounding a shaft, having a conductive ring element and a cylindrical spring with a diameter which is the same as that of the conductive ring element. The spring constant of the cylindrical spring is such that the spring keeps the conductive ring element within the housing in contact with the shaft and the housing.
DE 35 11 755 A1 discloses an arrangement for deflecting wave voltages, wherein on the shaft side of the electrodynamic machine, on which the bearings are isolated electrically with respect to ground, mass and foundation by insulating sections, a current path having a contact device with at least one sliding contact and a capacitance connected in series therewith is provided between the shaft and the machine mass or ground.
DE 60 2004 011 867 T2 discloses the reduction of wave voltages and wave currents by means of conductive bearing grease, insulation of the bearings and use of copper-phosphorus brushes and a Faraday shield. In addition, an annular frame in an annular channel having a plurality of electrically conductive threads can be used in order to reduce ionization in the presence of an electric field.
DE 10 2013 200 356 A1 discloses a bearing system for a wind turbine with an insulating ring for the electrical isolation of a bearing ring from a wind turbine element.
The teachings of the prior art are not sufficient to ensure the electromagnetic compatibility of a drive arrangement for an electrically driven vehicle.
The object of the invention is to devise a drive arrangement for an electrically driven vehicle which exhibits improved electromagnetic compatibility.
A drive arrangement for an electrically driven vehicle comprises an electric motor, which is designed to accelerate or decelerate the vehicle, wherein the electric motor has a rotor, which is arranged inside a stator having stator windings and is arranged on a rotor shaft. The drive arrangement further comprises a drive shaft, which is coupled to a drive wheel of the vehicle. The drive arrangement also comprises a shaft coupling, which transmits the torque output by the rotor shaft, such that said torque can be conducted to the drive shaft, wherein the shaft coupling has a torque receiving element, which receives the torque output by the electric motor, and a torque output element, which is mechanically coupled to the torque receiving element and outputs the torque in the direction of the drive shaft. An electrically insulating grease is arranged between the torque receiving element and the torque output element.
The effect of the electrically insulating grease is that the shaft voltage or the shaft current cannot propagate further via the shaft coupling and thus the electromagnetic compatibility of the drive arrangement is increased. The shaft currents and shaft voltages are not conducted into the gearbox and not into the drive shaft of the electrically driven vehicle.
It not necessary for the torque receiving element to be coupled directly to the rotor shaft. Nor is it necessary for the torque output element to be coupled directly to the drive shaft.
The torque receiving element and the torque output element can be connected to each other in a form-fitting manner. The insulating grease can be arranged on the form-fitting connection.
The torque receiving element and the torque output element can form a shaft-hub interface. The torque receiving element can be a toothed shaft and the torque output element can be a hub with a broached internal profile. As an alternative to this, the torque receiving element can be a hub with a broached internal profile and the torque output element can be a toothed shaft.
The electrically insulating grease can have particles of an electrically insulating solid. The electrically insulating grease can have polytetrafluoroethylene particles, polytetrafluoroethylene flakes or the like. Polytetrafluoroethylene is also known under the trade name Teflon. As a result of the addition of polytetrafluoroethylene as an additive to the grease, an insulating effect is achieved, since the polytetrafluoroethylene particles can be deposited on the torque receiving element and torque output element.
Preference is given to particles, flakes or additives which form an insulating film between and/or on the torque receiving element and the torque output element. The insulating film can, for example, be produced on account of the mechanical forces which act on the grease and the particles, the flakes or the additives.
A lubricating grease must be arranged on the torque receiving element and on the torque output element in order to prevent corrosion. As a result of adding the additive to the grease, both good corrosion prevention and good electrical insulation are achieved.
The electrically insulating grease arranged between the torque receiving element and the torque output element can have a contact resistance between the torque receiving element and the torque output element of at least 10 kΩ, preferably of at least 100 kΩ, most preferably of at least 1 MΩ. With a contact resistance of about 1 MΩ, the electromagnetic compatibility of the drive arrangement is improved by about the factor 10, which corresponds to damping of about 20 dB.
The torque receiving element can be formed integrally with the rotor shaft. For example, the rotor shaft can have a toothed shaft profile at one end or a hub profile with a broached internal profile. In this embodiment, the electrical isolation of the shaft currents or shaft voltages is carried out very close to the electric motor. The rotor shaft can be coupled to the drive shaft by a gearbox. The torque output element can be formed integrally with a gearbox shaft. The effect of this embodiment is that the drive arrangement is firstly compact and secondly the shaft currents or shaft voltages are isolated close to the electric motor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.